The present invention relates to a frequency synthesizer and, more particularly, to a frequency synthesizer used for a frequency modulation (FM) transceiver.
An FM transceiver which has multiple frequency channels needs a local oscillation frequency for reception and a carrier frequency for transmission. These frequencies have to be changed depending on the frequency appropriation for the channels. A frequency synthesizer having a phase locked loop (PLL) is often used as a source of high frequency oscillation adapted to such frequency change. One example of such frequency synthesizer is disclosed in the pending U.S. patent application Ser. No.157,797 by Y. Mizota, entitled "Radio Subscriber System", assigned to the present applicant and filed on June 9, 1980, now U.S. Pat. No. 4,373,205.
In a frequency synthesizer of this type, which will be described later in greater detail, the output of a reference oscillator is led to both a first oscillator circuit with the PLL for transmission and a second one similar to the first one for reception. Each oscillator circuit having the PLL includes a variable frequency divider, whose dividing ratio is varied with a channel designation signal, thereby to provide the transmission or reception frequency. The PLL-equipped oscillator circuit for transmission is used also as a modulator, in which a modulating signal is applied to its voltage-controlled oscillator (VCO) together with a control voltage representative of the frequency difference between the VCO output and the reference oscillation.
The frequency deviation .DELTA.f present in the above-mentioned VCO output within the PLL is given by: EQU .DELTA.f=K.sub.v .multidot.V.sub.m ( 1)
wherein K.sub.v (Hz/V) is the modulation sensitivity of the VCO and V.sub.m (V), the level of the modulating signal. Therefore, in order to achieve sufficiently linear and stable modulation, the modulation sensitivity K.sub.v is required to be linear over a wide range of the modulating signal voltage and stable over a broad frequency and temperature range.
The modulating sensitivity K.sub.v of a conventional FM transceiver of this type is not stable enough to satisfy these requirements, unless a complicated circuit structure is relied on.